You can easily get some very good nigiri on a budget of, say $30-50 but it's not going to be very stomach-filling. Kanoyama and Ushiwakamaru both offer high quality nigiri sets but you may not be satisfied after eating one, unless it's Ushi's 15 pieces for $50 deal.

Sushi On Hudson has very good, fresh fish. I have never eaten in the restaurant, but I get delivery from it all the time, and the price is among the lowest anywhere. I usually have the sashimi deluxe which has about 20 pieces for $14.95 including rice and soup or salad.